The advantages of using fiber optic communications networks include immunity to electro-magnetic interference, freedom from ground current loops, enhanced security and potential cost savings. The most common types of fibers used today in such networks have either a homogeneous stepped-index core and a cladding with significantly lower refractive index than the core so that some of the light in the core is reflected back into the core by a lossless process called total internal reflection or a gradient index core which similarly guides light by continuous refraction. In a single mode fiber, the core diameter and the core-to-cladding refractive index difference are kept sufficiently small so that only the lowest order mode is guided. In the multimode stepped index fiber, because the core diameter is larger and the refractive index difference is larger, many modes are guided. Graded index fibers are multimode fibers with a core whose refractive index decreases almost parabolically with radius.
Laser diodes are often coupled to multimode optical fibers for data transmission purposes. Such laser diodes emit from a single spot or point source which is .about.1 .mu.m high by .about.2-8 .mu.m wide. The core region of a multimode optical fiber is generally 50-500 .mu.m in diameter. Light from the laser is generally launched into the fiber by positioning the small laser spot in the center of the core of the fiber and positioning the emitting region of the laser very close to the end of the fiber. With this coupling method, a uniform distribution of light in all spatial modes of the multimode fiber occurs only after the light has traveled a considerable distance in the optical fiber. If light in the fiber is split in a STAR coupler, a non-uniform light distribution in the fiber will result in a non-uniform light distribution in the output fibers of the STAR coupler. Thus it is important to distribute the light as uniformly as possible at the fiber input.
Modal noise is the name given to an undesired modulation of the light intensity emerging from a multimode optical fiber. Basically, such noise is caused by the interference of the light with itself in a time-varying manner. Under certain conditions (high source coherence plus either fiber motion, temperature changes, or source wavelength charge), a speckled intensity pattern which changes with time exists at the fiber output plane. If in addition some form of mode-selective attenuation is present modal noise is high when a highly coherent light source, such as a single longitudinal mode semiconductor laser, is used as the input to the fiber since, due to the long coherence length of the light output of a single longitudinal mode laser, the light can propagate a long distance through the fiber and still interfere with itself. More specifically, it is known that there exists a critical frequency difference, .DELTA..nu..sub.C, which is a function of the total amount of modal dispersion in the fiber link. When multiple source frequencies differ from one another by less than .DELTA..nu..sub.C, they will all interfere with one another at the fiber output plane, creating a speckle pattern of high contrast, and thus creating the opportunity for modal noise. When there are N optical sources, all of which differ from one another by more than .DELTA..nu..sub.C, interference is eliminated and the N independent speckle patterns sum intensity-wise, reducing the speckle pattern's contrast to 1/.sqroot.N. Contrast reduction implies modal noise reduction. Thus it is clear that a large number of source frequencies, all separated by more than .DELTA. .nu..sub.C, is desired to minimize modal noise.
An ideal light source for a multimode fiber from the reduction of modal noise standpoint and from the standpoint of uniformly illuminating a fiber is a light-emitting diode (LED). Since modal noise is related to coherence length and an LED is an incoherent emitter, modal noise is eliminated. Additionally, an LED is an extended spatial source which more uniformly illuminates most of the modes of the fiber. However, an LED cannot be modulated as rapidly as a laser, nor can it inject as much optical power as can a laser, an important consideration when power splitters are to be used. In addition, because of its broad emission spectra, fiber wavelength dispersion limits the system bandwidth to undesirably low data rates.